Improper internal pressure of a tire causes a drop in running safety of the vehicle. Therefore, there have been methods for monitoring the internal pressure of moving tires by the use of pressure sensors. A problem with them, however, has been the expensive price of the pressure sensors, which can run up the cost of the monitoring system.
In response to it, there are methods proposed for estimating the internal pressure of a tire based on load signals produced by measuring the load with a load measuring apparatus incorporated into the wheel bearing (see Patent Document 1, for instance).
Also, there are methods proposed for detecting the forces working on the tread from the road surface using a sheet-like surface pressure sensor having substantially the same width as the tread of the tire which consists of a plurality of piezoelectric elements arranged in a matrix inside the tread. This surface pressure sensor provided over the whole circumference of a tire can measure the pressure distribution on the surface of the tire tread, thus enabling accurate estimation of the contact patch shape of the tire (see Patent Document 2, for instance).